Laser diodes emit a diffraction limited beam in a fast direction, but typically emit wide multimode beams in a slow direction. For many applications, it is desirable that laser diodes also emit a diffraction limited beam in a slow direction.
Most laser diodes up till now are edge emitters, that is, the beam exists in the semiconductor waveguide layers and exits from the cleaved edge of the processed laser diode chip. These are also called Fabry-Perot diode lasers since the cavity is essentially similar to that of a conventional gas or solid state laser but formed inside the semiconductor laser diode chip itself. The mirrors are either formed by the cleaved edges (also called “facets”) of the chip or (for high performance types like those that are very stable or tunable) one or both of these are anti-reflection coated and external mirrors are added.
Vertical cavity surface emitting laser diodes, on the other hand, emit their beam from their top surface (and potentially bottom surface as well). A cavity may be formed of a hundred or more layers consisting of mirrors and active laser semiconductor, and all may be formed epitaxially on a bulk (inactive) substrate, which in some cases is etched away in the vicinity of the laser cavity.
There are also optically pumped semiconductor lasers. Nearly all semiconductor lasers are powered by electrical current through the gain medium. However, for certain materials and device configurations, it is also possible to use another laser to optically pump its gain medium. This has some significant advantages in terms of controlling transverse and longitudinal modes and beam shape.
Vertical extended cavity surface emitting lasers are known. They are very similar to the optically pumped semiconductor laser, but use electrical pumping similar to a conventional edge-emitting laser diode or vertical cavity surface emitting laser diodes. However, the extended cavity allows frequency doubling or other nonlinear processes to be performed within the laser resonator, much like in the optically pumped semiconductor laser.